The present invention relates to fault detection systems for photovoltaic module systems.
Solar or photovoltaic (PV) energy conversion is presently used in a wide range of applications. Typically, a number of solar cells are connected together to provide practical voltage (for example, 5 to 15 VDC) and power output levels and are packaged in a protective enclosure to form a PV module. PV modules are available in many shapes and sizes from number of commercial sources.
Intermediate sector photovoltaic power systems, i.e., those with power ratings to several megawatts, are typically utility interactive. For these systems, power efficiency dictates DC bus voltages in the range of 400-1000 VDC. That requires a large number of 5 to 15 VDC photovoltaic modules arranged in series-connected strings.
A photovoltaic module within a series string has two basic failure modes, "short/bypass" and "open". A module shorted or in bypass contributes no power. The affected string, with one or more of its series voltage generating elements malfunctioned, is unable to deliver full, or possibly any power to the bus. A practical diagnostic procedure involving sequential and selective isolation masking exists for identifying the location of a module with that type of failure.
The second failure mode is characterized by a complete loss of power from the string, i.e., an infinite string impedance as the result of an open circuit or total disconnect. The system power loss in the event of an open circuit is a function of both the number of modules in series as well as their output current. In one system in use today, the Georgetown University PHENEF system rated at 300 KWpk, there are 124 bipolar strings of two monopoles each (plus and minus). Each monopole in turn consists of 18 series-connected PV modules rated at 72 watts, each module measuring two feet by four feet. The modules are flat units which are mounted onto the roof of the building. For this system, loss of a module due to an open circuit causes loss of the output of the entire monopole string, resulting in a peak output power loss for the entire system of about 0.4%.
The consequence of repair on a "walk-on" photovoltaic roof structure, without first having identified the location of the fault, can be serious, because the photovoltaic modules actually form part of the roof structure. In the Georgetown example, 4464 photovoltaic modules actually form the curtain-wall-type structure similar to an atrium glass roof. Up to 18 modules in the string might have to be removed from their sealing frames, and additionally, an equal number of adjacent modules would be disturbed. There is also a real probability of introducing roof leaks.
In circumstances where the back side of the array is accessible and the module interconnections can be conveniently reached from the rear (i.e., shaded side) of the modules, open circuit fault isolation can be accomplished by progressive check of the jumpers, the module receptacles and the mating plugs. As the array dimensions grow, even this direct contact method becomes more unsafe and time consuming because the maintenance technician is required to make checks when potentially hazardous voltages and currents are being generated, i.e., when there is solar radiation incident on the modules. The method also unnecessarily exercises the connectors, which could lead to unreliability.
For array elements installed directly on existing roofs (and with inaccessible intra-string wiring), and for installations where the modules form the final roof above a subroof, no suitable technique is available for pinpointing the location of open circuit faults within a module string. In such a "back sealed" (back inaccessible) roof-top installation, it is virtually impossible to precisely locate an "open" module fault, or a module-to-module open circuit without physically removing all or part of an entire module string. This is costly, results in loss of energy conversion availability, (i.e., "downtime") and can lead to secondary failures.
One solution to the problem may be the use of fault indicators or test points which are built into the modules. That solution would increase the cost of the modules, would be susceptible to failure, and would drain power from the system, thereby reducing its efficiency.